Fix Issue 18433 - rdmd doesn't respect DFLAGS for its cache hash

rdmd.d

@@ -231,7 +231,8 @@ int main(string[] args)
     }
 
     // Compute the object directory and ensure it exists
-    immutable workDir = getWorkPath(root, compilerFlags);
+    auto environmentFlags = environment.get("DFLAGS", "").split(" ");
+    immutable workDir = getWorkPath(root, compilerFlags ~ environmentFlags);
     lockWorkPath(workDir); // will be released by the OS on process exit
     string objDir = buildPath(workDir, "objs");
     Filesystem.mkdirRecurseIfLive(objDir);

rdmd_test.d

@@ -637,6 +637,27 @@ SHELL = %s
         assert(res.status == 0, res.output);
         assert(std.file.read(textOutput) == "hello world\n");
     }
+
+    // https://issues.dlang.org/show_bug.cgi?id=18423
+    // rdmd ignores changes in DFLAGS
+    {
+        string srcName = tempDir().buildPath("force_rebuild_dflags.d");
+        std.file.write(srcName, `void main() { import std.stdio; version(Foo) {"foo".writeln;} else { "bar".writeln; }}`);
+
+        res = execute(rdmdArgs ~ [srcName]);
+        assert(res.status == 0, res.output);
+        assert(res.output.canFind("bar"));
+
+        environment["DFLAGS"] = "-version=Foo";
+        res = execute(rdmdArgs ~ [srcName]);
+        assert(res.status == 0, res.output);
+        assert(res.output.canFind("foo"), res.output);
+
+        environment.remove("DFLAGS");
+        res = execute(rdmdArgs ~ [srcName]);
+        assert(res.status == 0, res.output);
+        assert(res.output.canFind("bar"));
+    }
 }
 
 void runConcurrencyTest(string rdmdApp, string compiler, string model)